1. Field of the Invention
The present invention relates to a method for driving a phase transition type liquid crystal display device. More specifically, it relates to such a method that realizes gradational display, as well as binary display, e.g., bright and dark display, in the liquid crystal display device comprising a nematic-cholesteric (cholesteric-nematic) phase transition type liquid crystal display panel, which allows a bright and stable screen display over a large image area and is capable of writing and erasing image information because of the memory effect and further promotes compactibility.
2. Description of the Related Art
Generally, liquid crystal display devices, utilizing conventional kinds of liquid crystals, such as twisted nematic (TN) liquid crystals and super twisted nematic (STN) liquid crystals, have such advantages that they have a thin and passive-type construction and are light weight and require relatively low power consumption. Due to the above advantages, the conditions that are necessary for panel type or portable type display devices, can be practically satisfied in the above liquid crystal display devices. Accordingly, they have attracted public attention in various regions. Particularly, in recent years, the performance of the above mentioned liquid crystal display device has improved significantly and the cost thereof has decreased significantly with the development of electronics and computer systems and therefore they have been widely used for office automation (OA) systems, domestic electric apparatuses and wrist watches, etc.
On the other hand, projection type liquid crystal devices such as over head projectors (OHP), including the liquid crystal display panels capable of a screen display over a large image area, are now in demand and come into practical use in some areas other than those of the former TN and STN liquid crystal display devices.
Among the latter liquid crystal display devices, the liquid crystal display device utilizing nematic-cholesteric (N-C) phase transition type liquid crystal have a memory function in which the information can be stored by selectively driving the liquid crystal with the driving voltage. If the bistable drive between bright conditions and dark conditions is executed for the above N-C phase transition type liquid crystal, by virtue of its memory function, a brighter and flicker-free image having large amounts of picture elements and an high resolution can be attained on a large screen. Therefore, the above liquid crystal display device can be expected to be applied to terminals for presentation at educational conferences, public notices and the like, as a projection type display device. In other words, it is suitable to be used when many persons discuss a particular topic while simultaneously viewing a screen {see, for example, Japanese Unexamined Patent Publication No. 61-60782 (the corresponding U.S. application is copending) and No. 61-198270 (corresponding to U.S. Pat. No. 4812034)}.
Generally, the above mentioned N-C phase transition type liquid crystal has a hysteresis curve in the light transmittance characteristic in relation to the driving voltage applied to the liquid crystal, different from the characteristic of TN liquid crystal and STN liquid crystal. Typically, when the voltage is relatively low, the liquid crystal is in a cholesteric phase, i.e., focal-conic (F) state in which the incident light is scattered in the liquid crystal and is difficult to be transmitted. Next, even when the voltage is increased, the above liquid crystal still remains in a focal-conic (F') state similar to the F state as long as the voltage is below the threshold voltage (V.sub.CN) from the cholesteric phase to the nematic phase. Further; when the voltage is above V.sub.CN, the light transmittance of the liquid crystal increases rapidly and soon the liquid crystal reaches the nematic phase, i.e., transparent homeotropic (H) state in which the light transmittance thereof is sufficiently high so that the incident light may be substantially transmitted. Conversely, even when the voltage is decreased gradually from the initial voltage for the H state of the liquid crystal, it remains in an homeotropic (H') state similar to the H state as long as the voltage is above the threshold voltage V.sub.NC from the nematic phase to the cholesteric phase, which is usually lower than the former threshold voltage V.sub.CN. When the voltage is below the V.sub.NC, the liquid crystal reaches the F state again. As explained above, when the driving voltage applied to the N-C phase transition type liquid crystal is increased and decreased, the light transmittance thereof changes, passing through the respective corresponding routes different from each other, namely, making a hysteresis curve. In this case, the above liquid crystal attains two stable states, i.e., H (H') state and F (F') state, corresponding to the bright state and the dark state respectively, for an applied voltage between the V.sub.NC and V.sub.CN, for example the sustaining voltage V.sub.d described later. Therefore, the stable binary display between the dark state and the bright state with a storage mode can be attained in an N-C phase transition type liquid crystal.
Here, the process for fabricating a liquid crystal display device using the above liquid crystal will be described in detail. First, a couple of transparent substrates, e.g., glass substrates are provided. Next, plural lines of transparent striped electrodes made of transparent conductive films, e.g., indium tin oxide (ITO) films, are formed on the above substrates respectively. One of the above two groups of striped electrodes are called scanning electrodes, while the other electrodes are called signal electrodes. Further, the alignment coatings that make the liquid crystal align to the fixed direction are formed on the two groups of electrodes respectively, and then a spacer of about 10 .mu.m thickness is held between the two substrates. If the N-C phase transition type liquid crystal is filled and sealed in an enclosed space formed by the above spacer, an N-C phase transition type liquid crystal display panel having the above mentioned hysteresis characteristic is finished. The picture elements are formed in the above liquid crystal display panel, corresponding to all the intersections between scanning electrodes and signal electrodes. If the appropriate voltage is applied by the drivers between two groups of electrodes selectively (in the form of an image), a desired image is formed on the display panel by means of the above picture elements. If the display panel is combined with an optical system composed of a light source, condensing lens and screen, etc., the above desired image is enlarged and projected on the screen. By the above mentioned process, a projection type liquid crystal display device comprising an N-C phase transition type liquid crystal display device is fabricated.
This N-C phase transition type liquid crystal display device does not need a polarization plate, different from the TN liquid crystal display device and the STN liquid crystal display device, etc. Accordingly, the former display device has the following advantages:
First, the image on the screen becomes brighter and an excellent image contrast can be attained.
Second, a temperature rise of the liquid crystal due to absorption of light of the polarization plate can be avoided, and therefore the characteristics of the liquid crystal do not deteriorate.
Furthermore, the N-C phase transition type liquid crystal display device is operative to control the scattering of light, not the phase lead or phase log. Therefore, the light, which does not reach the condensing lens of the OHP because of the scattering, is not finally projected on the screen. Consequently, the image contrast on the screen can be much better than that of the TN and STN liquid crystal.
However, as explained above, the N-C phase transition type liquid display device executes only a bistable drive of H (bright) state and F (dark) state utilizing hysteresis characteristics. In other words, the light transmittance of the above liquid crystal display device does not correspond on a one to one basis in relation to the driving voltage, different from that of the TN and STN liquid crystal display device. Therefore, the middle state between the H state and F state, i.e., middle brightness is difficult to display with good reproducibility. A gradational display by controlling the above middle brightness with function higher than a binary display of bright and dark states is likely to be required by various users. Thus, the development of the N-C phase transition type liquid display device that allows a gradational display is desired.